The present invention relates to a DNA probe for identifying bacterial strains of the Lactobacillus helveticus species, a process for producing such a probe and a method for identifying bacterial strains of this species with this DNA probe.
Lactobacillus helveticus is a very important bacteria for the fermentation of food. It is predominantly used in the fermentation of milk products and is found in starter cultures for some cheese and, in some countries, also for yoghurt production. Fermentation and maturation of these food products usually result from growth association and interaction of different bacteria, in most cases different Lactobacilli, Lactococci and other bacterial species. As most of these bacterial species have very similar nutritional requirements and grow under similar environmental conditions, a clear identification within the Lactobacillus species is sometimes very difficult. In particular, it may be difficult to differentiate between the two yoghurt bacteria L. bulgaricus, which belongs to the species of L. delbrueckii, and L. helveticus (formerly also called L. jugurti; O. Kandler and N. Weiss, Bergey's Manual of Systematic Bacteriology, vol. 2, 1208-1260, 1986). So far the classification of all these species is tedious and involves unreliable criteria, like sugar fermentation patterns and acid production. Due to these tests an identification of these different species remains doubtful and often arbitrary.
DNA hybridisation techniques, using specific DNA probes, are a very valuable tool for the identification of bacterial and viral strains, and have already found application in clinical diagnostics. Such DNA probes have already been used for the identification of Yersinia enterocolitica (J. Jagow et al., Appl. Environ. Microbiol. 51, 441-443, 1986), Plasmodium falciparum (R. H. Barker et al., Science 231, 1434-1436, 1986), Salmonella typhi (F. A. Rubin et al., J. Clin. Microbiol. 22, 600-605, 1985), Bacillus subtilis (J. Krauss et al., FEMS Microbiol. Lett. 33, 89-93, 1986), Haemophilus influenzae (F. Malouin et al., J. Clin. Microbiol. 26, 2132-2138, 1988) and other bacterial species, of DNA viruses (J. Brandsma et al., Proc. Natl. Acad. Sci. USA, 77, 6851-6855; P. Stolhandske et al., J. Clin. Microbiol. 15, 744-747, 1982; P. Stolhandske et al., J. Med. Virol. 12, 213-218, 1985), as well as RNA viruses (J. Flores et al., Lanceti, 555-559, 1983; M. Lin et al., J. Virol. 55, 509-512, 1985).
In the species of Lactobacillus, DNA probes for L. curvatus (H. A. R. Petrick et al., Appl. Environ. Microbiol. 54, 405-408, 1988,) which is specifically associated with spoilage of vacuum-packed meat, and for the L. delbrueckii species, comprising L. bulgaricus, L. lactis and L. delbrueckii (M. Delley et al., submitted), have been isolated. It would be of use in the dairy industry not only to be able to quickly identify L. delbrueckii, as can be done by the procedure described in our European Patent Application No. 89106016.2, but also to have a quick and reliable method to detect and classify strains belonging to the L. helveticus species.